Controlling droplet size
Developers of nasal spray products tune an array of variables, some relating to the formulation others to the device, to achieve
a target droplet size profile. Nasal spray devices usually incorporate a manually driven spray pump, which when actuated by
the patient, pushes the liquid formulation through an orifice, thereby applying energy for atomisation (Figure 1). Aspects of spray pump design, such as the precompression ratio, and the geometry, length and orifice size of the actuator
determine the shear force applied during use, which, in turn, influences the size of the droplets produced. The number and
size of doses for which the product is intended will also be taken into account when determining the design of the device.
Figure 1: Schematic of a nasal spray device.
In terms of the formulation, modifying properties by changing the composition alters the response to conditions imposed by
the device. A product may be solution- or suspension-based, with excipients levels controlled to meet atomisation and stability
requirements. Viscosity modifiers such as glycerin, polyvinylpyrrolidone (PVP) and various cellulose-derivatives are especially
common because the viscosity of the formulation has a marked impact on behaviour, with more viscous liquids requiring greater
energy for dispersion to the same droplet size. Surface tension is also an important physical property and may affect droplet
Developing an optimal product relies on the effective manipulation of these influential variables, but this is only possible
with reference to relevant droplet size data. Laser diffraction meets this need by providing real-time particle size analysis
through a spray event.
Using laser diffraction for nasal spray analysis
With laser diffraction, the size of droplets in a spray is determined from the scattering pattern produced as particles pass
through a collimated beam of light. Smaller particles scatter light weakly at wider angles, while larger particles produce
a stronger signal at narrower angles. The technique is non-destructive, requires no calibration and is suitable for even concentrated
sprays, providing that appropriate mathematical algorithms are used to analyse the scattered light pattern.
To be effective for spray measurement, laser diffraction systems must meet some essential criteria, including the ability
to rapidly acquire data to track the droplet size of a spray as it evolves during a single actuation of the device. For nasal
sprays, a wide dynamic range is needed so that the very large droplets (up to 600 µm in size) delivered at the beginning and
end of a spray pump actuation can be detected, along with any fines. This enables researchers to understand each phase of
atomisation during actuation, aiding the process of product optimisation.